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. 2022 Oct 28;49(21):e2022GL099154. doi: 10.1029/2022GL099154

Chromium Cycling in Redox‐Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications

David J Janssen 1,2,3,✉, Jörg Rickli 1,2,4, Martin Wille 1, Oscar Sepúlveda Steiner 3, Hendrik Vogel 1,2, Olaf Dellwig 5, Jasmine S Berg 6, Damien Bouffard 3,6, Mark A Lever 7,8, Christel S Hassler 9,10, Samuel L Jaccard 1,2,10
PMCID: PMC9787902  PMID: 36589775

Abstract

Chromium stable isotope composition (δ53Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ53Cr have not been assessed in modern redox‐stratified basins. We present new chromium (Cr) concentration and δ53Cr data in dissolved, sinking particulate, and sediment samples from the redox‐stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non‐quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox‐stratified basins show analogous patterns. This challenges assumptions from δ53Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non‐quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ53Cr from oxidative continental weathering.

Keywords: chromium, paleoproxy, stable isotopes, euxinia

Key Points

  • Non‐quantitative Cr removal at the chemocline drives chromium (Cr) isotope fractionation and the accumulation of light Cr in euxinic deep waters

  • Authigenic sediment δ53Cr differs from overlying water, contrary to paleoproxy application assumptions, and is similar to continental crust

  • δ53Cr signals in these settings may therefore reflect internal redox processes and not fractionation due to oxidative subaerial weathering

1. Introduction

Authigenic chromium (Cr) enrichments and Cr stable isotope (δ53Cr) composition in sedimentary deposits are widely used to trace redox conditions throughout Earth's history (Frei et al., 2009; Planavsky et al., 2014; Reinhard et al., 2013; Wei et al., 2020). These applications, which interpret high δ53Cr to signal elevated oxidative weathering of the continents, are based on the redox control of Cr solubility and stable isotope fractionation in aqueous environments. Reduced Cr (Cr(III)) is particle reactive and can be readily removed from the water column (Richard & Bourg, 1991), while oxidized Cr(VI) is soluble, with isotope fractionation resulting in enrichments of isotopically light Cr in Cr(III) (Ellis et al., 2002; Wanner & Sonnenthal, 2013). As a consequence of Cr reduction and removal, natural systems with strong O2 deficiency are associated with dissolved Cr depletions as well as elevated dissolved δ53Cr (Huang et al., 2021; Moos et al., 2020; Murray et al., 1983; Nasemann et al., 2020; Rue et al., 1997).

Paleo‐reconstructions using sedimentary δ53Cr records apply the assumption that Cr is efficiently sequestered into sediment phases under reducing conditions (anoxia or euxinia). Thus, complete (i.e., quantitative) Cr redox conversion prevents isotopic fractionation, resulting in sediment‐hosted authigenic δ53Cr equivalent to the overlying water column (Frei et al., 2009, 2020; Reinhard et al., 2013, 2014; Wei et al., 2020). However, the few studies of euxinic waters so far indicate the opposite, that is, enrichment rather than removal of dissolved Cr(III) (Achterberg et al., 1997; Davidson et al., 2020; Emerson et al., 1979), and no data from co‐localized euxinic waters and sediments are available. Due to the lack of data on Cr partitioning across redox interfaces and into underlying sediments, isotopic offsets resulting from partial (i.e., non‐quantitative) Cr(VI) reduction during removal to sediments have been neglected in δ53Cr paleoproxy interpretations.

Redox‐stratified basins are valuable settings to build the geochemical understandings necessary for developing and interpreting paleoproxies for the early stratified oceans. One such system, the meromictic alpine Lake Cadagno (Switzerland) (Figures 1a and 1b), has been used extensively as an analog for the Phanerozoic and Proterozoic oceans (Canfield et al., 2010; Dahl et al., 2010; Ellwood et al., 2019; Xiong et al., 2019) due to the intermediate sulfate levels and sulfidic bottom waters supporting anoxygenic phototrophs in the chemocline (Tonolla et al., 2003). To mechanistically constrain Cr cycling across redox gradients and incorporate these into the Cr‐based paleoproxy framework, we present [Cr] and δ53Cr in the water column (total dissolved) and sediments (near‐total digests and leachates), along with sinking particulates. These observations question fundamental assumptions inherent to the δ53Cr paleoproxy applications and will help to inform future δ53Cr‐based paleo‐reconstructions.

Figure 1.

Figure 1

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Water column data. (a): [O2], [Ca] and [Mg]. (b): map showing the location of Lake Cadagno. (c): dissolved [Cr], [Fe] and [Mn]. (d): dissolved and sediment trap δ53Cr (uncorrected and corrected for detrital chromium (Cr), see Section S.5 in Supporting Information S1). (e): Sediment trap Cr, Fe and Mn fluxes (note different depth range). The orange box in (a and c–e) indicates the chemocline. The gray box in (d) indicates average detrital δ53Cr (Schoenberg et al., 2008).

2. Study Area and Methods

2.1. Study Area

Lake Cadagno, at 1921 m elevation, is a 21 m deep meromictic alpine lake in Switzerland characterized by a permanent chemocline near 13 m depth. The initially oxic lake formed ∼13,500 YBP. A transition phase persisted between 10,00 and 9,000 YBP, followed by euxinic monimolimnion conditions, which have been generally stable until present (Berg et al., 2022). The lake's distinct geochemistry and microbial communities have been well‐characterized. Waters below the chemocline are fed by groundwater input from a karstic system composed of dolomite and gypsum, and are therefore rich in Ca2+, Mg2+, SO4 2− and HCO3 − relative to overlying waters (Del Don et al., 2001, Figure 1). In anoxic deep waters, dissolved concentrations reach 102 μmol kg−1 sulfide, ∼3.5 mmol kg−1 sulfate, and ∼1 μmol kg−1 Fe (e.g., Ellwood et al., 2019, see supplement). High densities of anoxygenic photoautotrophic green and purple sulfur bacteria are found at the chemocline in summer (Tonolla et al., 2003). These bacteria exert control on geochemical gradients in this zone (e.g., S, Fe; Del Don et al., 2001; Berg et al., 2016) and can form a 0.3–1.2 m thick mixed layer through bioconvection during summer (Sepúlveda Steiner et al., 2019, 2021; Sommer et al., 2017).

2.2. Sampling, Sample Purification and Analysis

Dissolved Lake Cadagno water samples were collected on 28–29 August 2017 from a floating platform at the deepest part of the lake. Sediment traps were deployed on 10 July 2017 and recovered on 6 September 2017. Sediments were sampled in summer 2019 and summer 2020 (Berg et al., 2022), freeze dried, and hand milled with an agate mortar and pestle. Baltic Sea samples were collected in the central Landsort Deep (site LD1; 435 m water depth, Häusler et al., 2018) onboard RV Poseidon (POS507, 29 October 2016).

Water column sampling and spiking (with a 50Cr‐54Cr double spike) followed standard procedures, and are discussed in Section S.1 in Supporting Information S1. Samples were processed through three stages of column chromatography: (i) Fe removal using AG1‐X8 resin in 6.4 M HCl (Scheiderich et al., 2015), followed by (ii) anion and (iii) cation chromatography as described elsewhere (Janssen et al., 2020; Nasemann et al., 2020; Rickli et al., 2019). Sediment near‐total digests were prepared using inverse aqua regia with H2O2. Authigenic sediment phases (primarily organic matter, potentially also sulfides, Figure 2a) were targeted with a 30% v/v H2O2 leach at pH = 2 following Rauret et al. (1999) (see Section S.1 in Supporting Information S1). Leach and digest subsamples were spiked with a 50Cr‐54Cr double spike, dried and processed through steps (i) and (iii). Reagents were either sub‐boiling distilled (acids) or Romil UpA and Fisher Optima grade (H2O2). Ancillary data are described in the supplement.

Figure 2.

Figure 2

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Near‐surface sediment data. (a): bulk sediment parameters (TOC, TS). (b): near‐total digest (CrBulk) and leach (CrAuth) concentrations, and CrAuth as % CrBulk. (c): δ53CrBulk and δ53CrAuth with average surface and deep water dissolved δ53Cr and sediment trap δ53Cr (total and detrital‐corrected), along with average detrital δ53Cr (gray box).

Sediment trap fluxes are calculated with and without corrections for lithogenic contributions (Figures 1 and 2, Table 1). Uncertainties on corrected data (pCrAuth) follow standard error propagation using lithogenic Cr/Al (Rudnick & Gao, 2014) and δ53Cr = −0.12 ± 0.1 ‰ (Schoenberg et al., 2008). Such corrections, which rely on normalizations to major crustal elements considered minimally mobile (e.g., Al), are complicated by the authigenic components of these elements in Lake Cadagno (Figures S6 and S7 in Supporting Information S1). Therefore, these corrections underestimate pCrAuth and overestimate δ53CrAuth, with true authigenic values lying between the corrected and uncorrected data (See Section S.5 in Supporting Information S1). Detrital corrections were not applied to sediment leach data as chemical leaches were more gentle than sediment trap digests (See Section S.1.2 in Supporting Information S1) enhancing the impact of authigenic phases of the normalizing element and exacerbating artifacts from improper corrections (Figures S6 and S7 in Supporting Information S1).

Table 1.

Sediment Trap, Diffusive and Burial Fluxes

Depth pCr pCrAuth
Downward fluxes m ng Cr ng Cr m−2 day−1 δ53Cr 2SEM ng Cr ng Cr m−2 day−1 δ53Cr 2SEM
Sediment Trap 10 3.2 × 103 4.2 × 103 −0.07 0.03 1.8 × 103 2.4 × 103 −0.03 0.09
Sediment Trap 14 7.0 × 103 9.2 × 103 −0.05 0.03 5.1 × 103 6.7 × 10 3 −0.02 0.05
Sediment Trap 20 5.4 × 103 7.2 × 103 −0.02 0.03 2.8 × 103 3.6 × 103 0.09 0.11
Euxinic zone [Cr] Chemocline base [Cr] Δ[Cr] ΔZ K Flux Flux
Upward Fluxes nmol kg⁻1 nmol kg⁻1 nmol cm⁻³ cm cm2 s‒1 nmol cm⁻2 s⁻1 ng m⁻2 d⁻1
Turbulent Diffusion 0.67 0.47 0.0002 150 1.1 × 10‒1 1.4 × 10−7 6.4 × 10 3
Molecular Diffusion 0.67 0.47 0.0002 150 3.4 × 10‒6 4.6 × 10−12 2.7 × 10−1
Sediment Acc. Rate1 Wet Sed. Density (ρ) Porosity (β) CrAuth FBurial FBurial FBurial
Cr burial mm yr−1 g cm−3 g H2O g wet sed.−1 ppm nmol cm‒2 y‒1 ng cm−2 y−1 ng m−2 d−1
Authigenic Cr Burial 4–6 2.30 0.33 8 0.9–1.4 49–74 1.3–2.0 × 103
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Note. Fluxes to and from the chemocline are in bold. Sediment trap data include both total (pCr) and detrital‐corrected (pCrAuth) fluxes (See Section S.5 in Supporting Information S1). Upward turbulent diffusive fluxes of dissolved [Cr] are based on [Cr] gradients from the chemocline to 1.5 m below the chemocline. 1Birch et al., 1996. See Section S.3.3 in Supporting Information S1 for further details on authigenic burial estimates.

Purified δ53Cr samples were dissolved in 0.7–1 mL 0.5 M HNO3 and analyzed with a Neptune Plus MC‐ICP‐MS (ThermoFisher) (Rickli et al., 2019). Internal sample uncertainty (2 SEM) and session reproducibility from NIST standards (n ≈ 10, 2 SD) is typically around 0.02–0.03 ‰. External uncertainty has previously been estimated at ±0.033 ‰ for full sample replicates (Janssen et al., 2020). Pure standard reference materials (Merck Cr(III) standard, this study: δ53Cr = −0.45 ± 0.03 ‰, n = 10; δ53Cr = −0.44 ± 0.02 ‰, Schoenberg et al., 2008) and USGS reference materials (Table S3 in Supporting Information S1) agree with published values.

3. Lake Cadagno Results and Discussion

3.1. Water Column

Dissolved [Cr] is stable in the surface mixed layer ([Cr] = 0.5 nmol kg−1, Figure 1c). A broad [Cr] minimum is found at 8–11 m depth, above the chemocline (∼12–13 m in summer 2017, Figure S1 in Supporting Information S1; Sepúlveda Steiner et al., 2021). Particulate Fe and Mn oxides form within and above the chemocline in Lake Cadagno, driven by upward transport of dissolved Fe(II)—a known Cr reductant (Richard & Bourg, 1991; Wanner & Sonnenthal, 2013)—and Mn(II) from the anoxic zone (Ellwood et al., 2019). As these oxides sink below the chemocline, they are reductively dissolved within the anoxic zone, and particulate Fe sulfides are formed (Ellwood et al., 2019). The [Cr] minimum lies within the most stratified portion of the water column (Figure S2 in Supporting Information S1) and corresponds to depths with enhanced formation and sinking of Fe and Mn oxides (see figure 5 in Ellwood et al., 2019). Therefore the [Cr] minimum is mechanistically consistent with Cr reduction coupled to Fe oxidation followed by Cr scavenging onto metal oxides.

Chromium concentrations begin increasing just above the chemocline, exceeding surface concentrations immediately below the chemocline. Dissolved [Fe] and [Mn] also increase over this range, with the [Cr] increase more closely mirroring [Fe] (Figure 1c). Matching [Cr], δ53Cr is stable in the surface mixed layer (δ53Cr = +0.86 ‰) (Figure 1d), with the isotopically heavy dissolved Cr signal likely reflecting surface water input to the lake, consistent with expectations from oxidative terrestrial weathering and global observations of riverine δ53Cr (Wei et al., 2020). δ53Cr decreases with depth, indicating the accumulation of isotopically light Cr in deep waters following Cr release during metal oxide reduction.

Sinking fluxes of authigenic particulate Cr (pCrAuth) increase near the chemocline, with maximum fluxes observed in the 14 m trap. Exported pCr is isotopically lighter than dissolved Cr, with Δ53Crpatriculate‐dissolved ≈ −0.6 ± 0.1 ‰ near the chemocline. This is comparable to net fractionations observed in other natural systems, but is much lower than theoretical values observed in lab studies (e.g., Wanner & Sonnenthal, 2013), which likely reflects incomplete removal of reduced Cr resulting in lower apparent fractionation factors for Cr reduction and removal (Huang et al., 2021; Nasemann et al., 2020; Wang, 2021). Isotopically, pCrAuth is indistinguishable from the lithogenic background (δ53Cr = −0.12 ± 0.10 ‰, Schoenberg et al., 2008). Export fluxes of pCrAuth and pMn decrease with depth in the euxinic zone (Figures 1e and Table 1; Table S2 in Supporting Information S1), while pFe species shift from oxide‐dominated above the chemocline to sulfides below (Berg et al., 2022; Ellwood et al., 2019), suggesting an Fe‐Mn‐oxyhydroxide shuttle and resulting in much of the Cr exported across the chemocline being released before the deepest sediment trap (20 m). The behavior of pCr is thus consistent with dissolved data indicating Cr release and accumulation in the euxinic zone following reduction of Fe and Mn oxides. Despite depth‐dependent variability in pCrAuth, exported δ53Cr is uniform within analytical uncertainty, indicating no fractionation during Cr release.

Deep layers of euxinic water bodies are assumed to remain quiescent given the strong stratification, and therefore the vertical gradient of [Cr] should be smoothed by an upward molecular diffusive flux. However, the pCrAuth export (∼6.7 × 103 ng Cr m−2 day−1) is orders of magnitude larger than the molecular flux of dissolved Cr (Table 1, Section S.3 in Supporting Information S1) while the observed [Cr] depletion is subtle. Despite the apparent quiescence of the deep interiors of lakes, various physical processes maintain a moderate and intermittent energetic structure (Saggio & Imberger, 2001), including in Lake Cadagno (Wüest, 1994). Here our concurrent microstructure observations indicate that vertical fluxes are sustained by turbulent diffusivity (Koc, Osborn & Cox, 1972), with a mean Koc in the layer of interest of 10−1 cm2 s⁻1 (Figure S2 in Supporting Information S1; four orders of magnitude larger than molecular diffusivity). This results in upward turbulent flux estimates comparable to sinking pCr fluxes, while the authigenic burial flux is comparatively smaller (∼25% of turbulent and particulate fluxes, Table 1). The background turbulence thereby explains Cr profiles and, particularly, the lack of a pronounced [Cr] minimum and local δ53Cr maximum above the chemocline, due to the substantial upward turbulent transport of isotopically light Cr. The result is similar to “cryptic” cycles, where rapid and localized redox cycling masks the expected biogeochemical signals (e.g., Berg et al., 2016), though rather as a larger‐scale physical‐geochemical transport cycle.

Observed deep water [Cr] enrichments likely reflect the integrated accumulation of Cr released from sinking particles and near‐surface sediments as well as the potential contribution from groundwater (Del Don et al., 2001), with concentrations further modified by turbulent mixing. As there is no modification of particulate δ53Cr with depth, released Cr must be of similar isotope composition as the particulates (δ53Cr ≈ 0 ‰). Therefore, any groundwater Cr must be isotopically heavier than 0 ‰ (see Section S.4 in Supporting Information S1). However, while subaquatic springs likely influence specific aspects of Lake Cadagno deep water [Cr] and δ53Cr, other euxinic basins exhibit similar large‐scale [Cr] and δ53Cr depth‐trends (see below). Therefore, a shared set of geochemical and physical controls likely shape the common large‐scale [Cr] and δ53Cr trends across diverse euxinic basins.

3.2. Sediments

Authigenic Cr (CrAuth) in near‐surface sediments is isotopically lighter than deep water, with similar δ53Cr as sinking particles (Figure 2). We do not observe strong [CrAuth] enrichments relative to [CrBulk], despite stably euxinic deep waters and large Cr fluxes from the chemocline, and CrAuth burial fluxes are small relative to chemocline particulate fluxes, reflecting significant Cr release from sinking particles (Table 1, Section S.3.3 in Supporting Information S1). Sediment Mn and Fe(III) content is low relative to sinking particles (Figure 1, Ellwood et al., 2019; Berg et al., 2022), supporting a Fe‐Mn‐oxyhydroxide shuttle for Cr, with Cr release following oxide dissolution. Bulk sediment δ53Cr (δ53CrBulk) is indistinguishable from average continental crust (−0.12 ± 0.10 ‰, Schoenberg et al., 2008) at all depths. δ53CrAuth is slightly higher than crustal signatures in the uppermost sediments; however, δ53CrAuth decreases with sediment depth and is indistinguishable from this inventory below 5 cm. δ53CrBulk and δ53CrAuth converge in the upper 10 cm of the sediment, suggesting modification of δ53Cr through early diagenesis (sedimentation rates ∼4–6 mm yr−1, Birch et al., 1996), similar to reports of Cr homogenization in black shales (Frank, et al., 2020).

While surface sediment δ53CrAuth equals sinking particulate δ53Cr, non‐quantitative water column Cr removal results in isotope fractionation and an offset between sediments and dissolved δ53Cr. This fractionation, along with potential early diagenetic modification, leads coincidentally to a δ53CrAuth equivalent to detrital reservoirs (Figure 2). δ53CrAuth is approximately 0.6 ‰ lower than dissolved δ53Cr in Lake Cadagno at the [Cr] minimum; however, variable dissolved δ53Cr results in inconsistent offsets between δ53CrAuth and surface, deep and chemocline dissolved δ53Cr. [CrBulk] and δ53CrBulk in sapropel samples, obtained from a sediment core spanning the last ∼12 ka (Berg et al., 2022) remain relatively stable with depth. δ53CrBulk is indistinguishable from lithogenic background and δ53CrAuth is indistinguishable from or only slightly heavier than lithogenic background (Figure S5 in Supporting Information S1). Thus, despite deposition in an anoxic basin below an oxidizing atmosphere, sediment δ53CrBulk and δ53CrAuth provide no isotopically heavy Cr record (reflecting oxidative weathering and high dissolved δ53Cr). These data imply that in other redox‐stratified settings, an isotopically heavy dissolved δ53Cr pool does not necessarily result in isotopically heavy δ53CrAuth or δ53CrBulk records, and that constraints on water column fractionation are needed to reconstruct dissolved δ53Cr of overlying waters and oxidative weathering processes from sedimentary δ53Cr records.

4. Synthesis of Redox‐Stratified Systems

δ53Cr has received significant attention as a paleoproxy for O2 availability, especially in the Proterozoic. While Lake Cadagno is a promising analogue for biogeochemical cycling in the Proterozoic ocean, it remains a small Alpine lake with unique physical and biogeochemical cycling. To assess the relevance of these data to other modern redox‐stratified systems and to oceanic systems throughout geologic time, we compiled new dissolved [Cr] and δ53Cr data from the weakly euxinic Landsort Deep site in the Baltic Sea (Häusler et al., 2018), together with published [Cr] data from other redox‐stratified basins (Figure 3).

Figure 3.

Figure 3

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Compilation of anoxic basin data with conceptual mechanistic diagram. Chromium, Fe (orange) and Mn (red) concentrations in the dissolved (open circles, dFe and dMn) and particulate (filled circles, pFe and pMn) phases from Lake Cadagno ((a), black, this study, pFe and pMn: Ellwood et al., 2019), Landsort Deep ((b), dark gray, this study), the Black Sea ((c), blue, Cr: Mugo, 1997, Fe and Mn: Lewis & Landing, 1991), and Saanich Inlet ((d), Emerson et al., 1979, light gray, only dissolved data). (e) combines (a–d) with Esthwaite Water (green, Achterberg et al., 1997) and Hall Lake (white, Balistrieri et al., 1994). In (a)–(f), the y‐axis ranges from the surface to sediments, with the chemocline at the center. In (a–e), [Cr] is normalized to surface concentrations. The orange shading in (a–d) indicates the zone of enhanced Fe and Mn redox cycling above the chemocline. (f–g) show a schematic of metal cycling in this zone, using Landsort Deep data (f) (see Table S8 in Supporting Information S1).

In the Landsort Deep, Cr is removed near the chemocline (Figure 3b, Table S7 in Supporting Information S1), coincident with a maximum in particulate Fe and Mn oxides. High dissolved [Cr] and low δ53Cr are found in underlying waters where dissolved [Fe] and [Mn] reach μM levels, indicating an accumulation of isotopically light Cr in euxinic waters as in Lake Cadagno. These data support isotope fractionation during non‐quantitative Cr reduction and removal via scavenging onto metal oxides slightly above the chemocline, with Cr release following oxide reduction in euxinic waters and/or surface sediments.

Data from other redox‐stratified basins are similar, despite wide ranges in dissolved H2S (10°–102 μmol kg−1) and Fe (10−2–102 μmol kg−1) concentrations and Fe/H2S (10−4–103) (Table S9 in Supporting Information S1). This includes Lake Cadagno and the Landsort Deep, Saanich Inlet (Emerson et al., 1979, BC Canada, see also Davidson et al., 2020), the Black Sea (Mugo, 1997), Esthwaite Water (UK, Achterberg et al., 1997) and Hall Lake (Wa USA; Balistrieri et al., 1994) (Figures 3a–3e). These can be more directly compared by normalizing profiles to [Cr]surface and relative depth above and below the chemocline, creating four quadrants: (I) [Cr] < [Cr]surface in oxic waters, (II) [Cr] > [Cr]surface in oxic waters, (III) [Cr] < [Cr]surface in anoxic waters, and (IV) [Cr] > [Cr]surface in anoxic waters (Figure 3e). All environments lie within (I) slightly above the chemocline, indicating non‐quantitative Cr removal. Anoxic deep waters fall into quadrant (IV) in most settings, indicating Cr accumulation rather than removal. Basins with only seasonal anoxia (Saanich Inlet, Esthwaite Water) differ, showing a slight increase in [Cr]Normalized just above the chemocline but remaining in zone (III) at depth. This may indicate insufficient time to allow Cr accumulation through the Fe‐Mn shuttle, due to their only seasonally anoxic nature.

This compilation from diverse permanently and seasonally anoxic systems confirms the general trends observed in Lake Cadagno—(i) non‐quantitative Cr removal above the chemocline (∼20–60% of [Cr]surface), suggesting isotope fractionation and the transport of low δ53Cr to anoxic waters, and (ii) Cr accumulation in anoxic deep waters indicating poor sequestration of Cr into sediments. Therefore, reconstructions of water column or weathering δ53Cr signals from sediments deposited in these environments require accounting for fractionation during Cr removal as well as internal cycling resulting in variable water column δ53Cr.

Chromium depletions consistently occur above the chemocline, coincident with elevated dissolved and particulate [Fe] and [Mn] (orange box in Figures 3a–3d). The sharpness of the Cr depletion above and subsequent increase below the chemocline can be explained by natural variability in the systems (see also Dellwig et al., 2012), including absolute depth ranges (10–103 m), the relative width of the Fe‐Mn redox zone (orange box height, Figures 3a–3d) and upward diffusive transport. Shallower systems (e.g., Hall and Cadagno Lakes), thicker Fe‐Mn redox zones, and elevated diffusivity correspond with broader Cr minima and more gradual Cr increases below the chemocline.

In agreement with previous studies (e.g., Balistrieri et al., 1994; Mugo, 1997), the consistent Cr minimum in the zone of intense Fe‐Mn redox cycling supports the widespread control of these metals on Cr removal in anoxic systems, whereby Cr reduction coupled to Fe oxidation and subsequent Cr(III) scavenging on particulate metal oxides drives Cr removal (Figures 3f and 3g), followed by oxide reduction and Cr release in the anoxic zone or near the sediment surface. The broadly similar Cr trends across these diverse freshwater and marine systems indicates that our data from Cadagno are generally relevant to other systems and can be used to revise the interpretational framework of δ53Cr paleoproxy applications. Specifically, these data indicate that:

  1. Cr is partially removed above the chemocline, but is not efficiently removed from the water column in anoxic systems, both at and below the chemocline

  2. Dissolved Cr generally accumulates in anoxic deep water

  3. Sediments from redox‐stratified basins may not always show high [CrAuth] enrichments

  4. Reconstructing water column δ53Cr from δ53CrAuth within these settings requires accounting for variable water column δ53Cr, fractionation during Cr removal and early diagenesis

  5. δ53CrAuth within these settings may therefore not directly reflect Cr fractionation originating from oxidative subaerial weathering

5. Conclusions and Implications

Available data across a range of redox‐stratified marine and lacustrine settings share fundamental features—namely, local [Cr] minima with non‐quantitative Cr removal slightly above the chemocline, and increasing [Cr] below—with permanently anoxic basins showing deep water dissolved Cr accumulation rather than efficient removal. Given the isotope fractionation associated with Cr reduction, non‐quantitative removal suggests that sedimentary authigenic δ53Cr should not match the water column, contrasting previous assumptions (e.g., Frei et al., 2009, 2020; Reinhard et al., 2013, 2014; Wei et al., 2020). Instead, sediment δ53CrAuth is isotopically offset from the water column, a factor that must be considered for paleoreconstructions. Furthermore, variable water column δ53Cr, as well as potential early diagenetic modification of δ53CrAuth, suggests there is no consistent offset between δ53CrAuth and water column δ53Cr. Indeed, our Lake Cadagno data show that sinking particulate δ53Cr is isotopically comparable to sediment δ53CrAuth, while dissolved δ53Cr decreases from surface waters to euxinic deep waters, and therefore dissolved δ53Cr differs from δ53CrAuth by variable extents.

Despite a strongly oxidizing atmosphere throughout the entire history of Lake Cadagno, we find no significantly fractionated sediment δ53CrAuth. Evidently, oxidative weathering does not necessarily result in high δ53CrAuth. To the contrary, the relatively small ranges of dissolved δ53Cr throughout modern systems (surface waters ≈ +0.2 to +1.2 ‰, Wei et al., 2020; Horner et al., 2021) coupled with effective fractionation for the reduction and removal of Cr (Δ53Cr ≈ −0.4 to −1.3 ‰; this study; Janssen et al., 2020, 2021; Moos et al., 2020; Nasemann et al., 2020; Huang et al., 2021; Wang, 2021) indicate that authigenic sedimentary δ53Cr records could easily be comparable to unfractionated continental crust (δ53Cr = −0.12 ± 0.10 ‰, Schoenberg et al., 2008). In other words, Cr removal from a water column inventory of +0.2 to +1.2 ‰, with a Δ53Crparticle‐dissolved of −0.4 to −1.3 ‰ is expected to yield an authigenic sedimentary δ53Cr that is, at times, within the range of −0.2 to 0.0 ‰. Consequently, caution should be used in the interpretation of δ53Cr records from redox‐stratified basins, which reflect local redox‐related fractionation processes superimposed on oxidative subaerial weathering signals.

Supporting information

Supporting Information S1

Click here for additional data file. (1.2MB, pdf)

Acknowledgments

The authors thank Nathalie Dubois for providing sediment traps and Alois Zwyssig for assistance deploying and recovering the traps. We thank the Piora Centro Biologia Alpina (CBA) for use of the sampling platform and housing as well as Samuele Roman for logistical support. We thank Julia Gajic for TC/TIC/TN/TS analyses, Patrick Kathriner for dissolved Ca and Mg analyses, and the crew and captain of RV Poseidon for assistance with Baltic Sea sampling. Elena Serra provided the map used in Figure 1. This manuscript was improved by comments from three anonymous reviewers. Funding was provided by the Swiss National Science Foundation (Grant PP00P2_172915) and the European Research Council (ERC CoG—SCrIPT Grant 819139) to SLJ. Chromium data were obtained on a Neptune MC‐ICP‐MS at the University of Bern acquired with funds from the NCCR PlanetS supported by SNSF Grant 51NF40‐141881. O.S.S. and D.B. were supported by SNSF Grant 179264 (BioCad). The specific contributions of each author are as follows: DJJ, JR and SLJ designed the study, with input from MW, HV and CSH. DJJ processed and analyzed all Cr samples. JR measured ancillary metal data for water column (dissolved and sediment trap) and sediment leach samples. JR, CSH and SLJ conducted Lake Cadagno water column sampling. JB, ML and HV coordinated sampling for the 8 m sediment core and MW and HV sampled the short core for near surface sediments. OD conducted Landsort Deep water column sampling and produced ancillary Landsort Deep data. MW conducted near‐total sediment digests and DJJ conducted sediment leaches. OSS and DB conducted physical limnological investigations of Lake Cadagno, coordinated multiparameter profile measurements presented in Figure S1 in Supporting Information S1, and derived turbulent diffusivity values. DJJ led interpretations of the data and preparation of the manuscript, with input from all co‐authors.

Janssen, D. J. , Rickli, J. , Wille, M. , Sepúlveda Steiner, O. , Vogel, H. , Dellwig, O. , et al. (2022). Chromium cycling in redox‐stratified basins challenges δ53Cr paleoredox proxy applications. Geophysical Research Letters, 49, e2022GL099154. 10.1029/2022GL099154

Data Availability Statement

Data are presented in the Supporting Information S1 and Table 1. Vertical microstructure data for turbulent diffusion estimations is available at https://doi.org/10.5281/zenodo.3507638. Chemical and CTD data are also available in the following open access datasets in the Zenodo repository: http://doi.org/10.5281/zenodo.7125831 and http://doi.org/10.5281/zenodo.7127882, respectively.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. Janssen, D. J. , Rickli, J. , Wille, M. , Sepúlveda Steiner, O. , Vogel, H. , Dellwig, O. , et al. (2022). Chemical data accompanying the manuscript “Chromium cycling in redox‐stratified basins challenges δ53Cr paleoredox proxy applications” in Geophysical Research Letters. (Version 1.0) [Dataset]. Zenodo. 10.5281/zenodo.7125831 [DOI] [PMC free article] [PubMed]
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Supplementary Materials

Supporting Information S1

Click here for additional data file. (1.2MB, pdf)

Data Availability Statement

Data are presented in the Supporting Information S1 and Table 1. Vertical microstructure data for turbulent diffusion estimations is available at https://doi.org/10.5281/zenodo.3507638. Chemical and CTD data are also available in the following open access datasets in the Zenodo repository: http://doi.org/10.5281/zenodo.7125831 and http://doi.org/10.5281/zenodo.7127882, respectively.

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